Uranium can be recovered from phosphoric acid by solvent extraction. In the first cycle of preferred processes using a di-2-ethylhexylphosphoric acid/trialkylphosphine oxide (D2EHPA/TOPO) solvent mixture, the uranium is reductively stripped from the solvent using phosphoric acid containing a high concentration of ferrous iron. The uranium can be recovered from this strip acid by first oxidizing the acid and then re-extracting the uranium, preferably with a D2EHPA/TOPO solvent mixture in a second cycle extraction. The uranium can be recovered from the second cycle solvent using an ammonium carbonate strip solution and a precipitation stage. This process is well known in the art, and is taught for example by Hurst et al., in U.S. Pat. No. 3,711,591; Elikan et al., in U.S. Pat. No. 3,966,873 and Sundar, in U.S. Pat. No. 4,002,716.
In this second cycle operation, the second cycle organic solvent, which is loaded with uranium after the extraction step, also contains entrained H.sub.3 PO.sub.4 and other impurities which must be removed. Hurst et al., Elikan et al., and Sundar, cited above, attempted to solve this problem solely with a 100% water wash or scrub between the second cycle extractor and the second cycle stripper. However, as H.sub.3 PO.sub.4 is removed by a 100% water wash, the phase disengagement in the scrubber becomes very poor. As a result, contaminated water containing H.sub.3 PO.sub.4 can be carried with the organic solvent to the stripper. The H.sub.3 PO.sub.4 may then be stripped by the ammonium carbonate and precipitated as a P.sub.2 O.sub.5 compound, and this can affect the purity of the final, recovered uranium material, especially at high commercial through rates.
Wiewiorowski et al., in U.S. Pat. No. 4,105,741, attempted to remove carbonate after a second cycle stripping step. There, a separate step of washing the second cycle solvent was used, after uranium stripping with carbonate. This required an outside stream of a purified acid selected from sulfuric, hydrochloric, nitric or iron-free phosphoric acid. The acid treated stream was then fed back to the second cycle extractors. However, this would not solve H.sub.3 PO.sub.4 entrainment problems which might occur prior to stripping.